This invention relates generally to circuits for providing high frequency energizing signals to electrical devices such as luminescent lamps and is an improvement on my U.S. Pat. No. 4,066,930 dated Jan. 3, 1978, incorporated herein by reference.
A standard measure of the efficiency of energy utilization in luminiscent sources is a parameter called "efficacy" which is the ratio of luminous flux output to the total power input. For example, the efficacy of present day fluorescent tubes is about 55 to 65 lumens per watt as compared to a figure of about 40 lumens per watt for typical incandescent lamps. Solely from the standpoint of energy utilization efficiency, therefore it is desirable to use fluorescent lamps for many lighting needs.
However, as relatively efficient as they are when compared with other light sources, present day fluorescent lamps fall far short of the efficiencies theoretically possible. Fluorescent lamps require a high voltage to initiate current flow across the lamp terminals and require a high current to initiate and to maintain ignition. This is due to the fact that there is an infinitely high impedance existing in the tube prior to ignition. Ignition occurs when the gases inside the tube are ionized permitting current to flow between the electrodes at opposite ends of the tube. Once a gaseous discharge tube has ignited, it exhibits a negative resistance characteristic and some form of current control device, such as a ballast, is typically utilized to limit the current to the tube.
Typically a fluorescent lamp ballast includes circuitry adapted to direct a high voltage (which may be as high as 1600 volts) to the gas tube electrodes. This high voltage is necessary in order to force electron emission from the electrodes and to thereby initiate ionization of the gases in the tubes. One or both of the electrodes generally comprises a filament which has the capacity of more readily emitting electrons when heated and subjected to high voltage and current.
One disadvantage with present day mercury, sodium vapor, and fluorescent lamp circuits is the loss of energy in the operation of the ballasts and in the heating of the filament electrodes. Another disadvantage is that the lifetime of the lamps is controlled principally by the mechanical integrity of the filaments. Once the filaments break and cease to emit electrons, a lamp no longer functions even though the light producing components of the lamp such as the gases in the tube and the phosphors on the tube walls remain functional. The present day ballasts continue to feed voltage and current into the system even though there is no live tube to effectively utilize it. This causes lamp flickering and overheating and can become extremely hazardous.
It is generally acknowledged that the energization of fluorescent tubes with high frequency signals is more effective and efficient than the standard ballast circuits. For one reason or another such as improper frequency circuit malfunctions in critical areas, excess radio frequency interference, or electromagnetic interference, however, these systems have not been commercially feasible. Apparently, in prior art circuits too much energy is lost in the switching and amplification of transistors and in the operation of the power transformer.
Another disadvantage of present circuits is the fact that bulb life is greatly reduced and the ends of the tubes tend to become blackened due to current distortions in the tubes caused by the introduction into the tubes of signals carrying too many harmonics.